How to capture the stdout/stderr of a unittest in a variable?

Question

How to capture the stdout/stderr of a unittest in a variable? I need to capture the entire output output of the following unit test and send it to SQS. I have tried this:

import unittest, io
from contextlib import redirect_stdout, redirect_stderr


class LogProcessorTests(unittest.TestCase):
    def setUp(self):
        self.var = 'this value'

    def test_var_value(self):
        with io.StringIO() as buf, redirect_stderr(buf):
            print('Running LogProcessor tests...')
            print('Inside test_var_value')
            self.assertEqual(self.var, 'that value')
            print('-----------------------')
            print(buf.getvalue())

However, it doesn't work and the following output appears only on stdout/stderr.

Testing started at 20:32 ...
/Users/myuser/Documents/virtualenvs/app-venv3/bin/python3 "/Applications/PyCharm CE.app/Contents/helpers/pycharm/_jb_unittest_runner.py" --path /Users/myuser/Documents/projects/application/LogProcessor/tests/test_processor_tests.py
Launching unittests with arguments python -m unittest /Users/myuser/Documents/projects/application/LogProcessor/tests/test_processor_tests.py in /Users/myuser/Documents/projects/application/LogProcessor/tests
Running LogProcessor tests...
Inside test_var_value

that value != this value

Expected :this value
Actual   :that value
<Click to see difference>

Traceback (most recent call last):
  File "/Applications/PyCharm CE.app/Contents/helpers/pycharm/teamcity/diff_tools.py", line 32, in _patched_equals
    old(self, first, second, msg)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/unittest/case.py", line 839, in assertEqual
    assertion_func(first, second, msg=msg)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/unittest/case.py", line 1220, in assertMultiLineEqual
    self.fail(self._formatMessage(msg, standardMsg))
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/unittest/case.py", line 680, in fail
    raise self.failureException(msg)
AssertionError: 'this value' != 'that value'
- this value
?   ^^
+ that value
?   ^^

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/unittest/case.py", line 59, in testPartExecutor
    yield
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/unittest/case.py", line 615, in run
    testMethod()
  File "/Users/myuser/Documents/projects/application/LogProcessor/tests/test_processor_tests.py", line 15, in test_var_value
    self.assertEqual(self.var, 'that value')



Ran 1 test in 0.004s

FAILED (failures=1)

Process finished with exit code 1

Assertion failed

Assertion failed

Any idea? Please let me know if more info is needed.

score 24 · Answer 1 · answered Apr 30 '20 at 22:03

Based on the contextlib.redirect_stdout documentation, this is how you'd redirect stderr or stdout:

import io
import contextlib

f = io.StringIO()
with contextlib.redirect_stderr(f):
    parser = target.parse_args([])
self.assertTrue("error: one of the arguments -p/--propagate -cu/--cleanup is required" in f.getvalue())

You can also combine that with another context manager (like assertRaises) like this:

f = io.StringIO()
with self.assertRaises(SystemExit) as cm, contextlib.redirect_stderr(f):
    parser = target.parse_args([])
self.assertEqual(cm.exception.code, 2)
self.assertTrue("error: one of the arguments -p/--propagate -cu/--cleanup is required" in f.getvalue())

score 16 · Accepted Answer · answered May 08 '19 at 21:21

If you manually instantiate the test runner (e.g. unittest.TextTestRunner), you can specify the (file) stream it writes to. By default this is sys.stderr, but you can use a StringIO instead. That will capture the output of the unittest itself. The output of your own print-statements will not be captured, but you can use the redirect_stdout context manager for that, using the same StringIO object.

Note that I would recommend to avoid using print-statements, since they will interfere with the output of the unittest framework (your test output will break the output lines of the unittest framework) and it's a bit of a hack to redirect the stdout/stderr streams. A better solution would be to use the logging module instead. You could then add a logging handler that writes all log messages into a StringIO for further processing (in your case: sending to SQS).

Below is example code based on your code using print-statements.

#!/usr/bin/env python3

import contextlib
import io
import unittest


class LogProcessorTests(unittest.TestCase):

    def setUp(self):
        self.var = 'this value'

    def test_var_value(self):
        print('Running LogProcessor tests...')
        print('Inside test_var_value')
        self.assertEqual(self.var, 'that value')
        print('-----------------------')


if __name__ == '__main__':
    # find all tests in this module
    import __main__
    suite = unittest.TestLoader().loadTestsFromModule(__main__)
    with io.StringIO() as buf:
        # run the tests
        with contextlib.redirect_stdout(buf):
            unittest.TextTestRunner(stream=buf).run(suite)
        # process (in this case: print) the results
        print('*** CAPTURED TEXT***:\n%s' % buf.getvalue())

This prints:

*** CAPTURED TEXT***:
Running LogProcessor tests...
Inside test_var_value
F
======================================================================
FAIL: test_var_value (__main__.LogProcessorTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "test.py", line 16, in test_var_value
    self.assertEqual(self.var, 'that value')
AssertionError: 'this value' != 'that value'
- this value
?   ^^
+ that value
?   ^^


----------------------------------------------------------------------
Ran 1 test in 0.000s

FAILED (failures=1)

This confirms all output (from the unittest framework and the testcase itself) were captured in the StringIO object.

Do you think it is possible to redirect stdout temporarily per TestCase such that one can make some assertions on the content of that buffer? — normanius, Nov 30 '19 at 20:28
@normanius, yes, I think that would be possible. Did you try it? If you've made an attempt but got stuck, please [ask a new question](https://stackoverflow.com/questions/ask) about it. — wovano, Dec 01 '19 at 13:16
Thanks. Figured out a way myself already. In case you have a better idea: [find here](https://stackoverflow.com/questions/59201313) the corresponding post. — normanius, Dec 05 '19 at 18:36

score 0 · Answer 3 · answered May 08 '19 at 17:17

0

Honestly the easiest way is probably to redirect your output at the OS level--run the test from the command line and > it to a file.

If you are using a build system to execute these, then the build system should be capturing the output for you and you can extract the output from it's build artifacts.

answered May 08 '19 at 17:17

Bill K

62,186
18
105
157

Thanks @BillK for the reply. I have to run these tests using a Lambda on-demand. So I don't think I can use `>`. But yeah, it would have been the simplest approach. – Bhushan May 08 '19 at 17:21

How to capture the stdout/stderr of a unittest in a variable?

3 Answers3

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